Monolithic White Light-Emitting Diode

ABSTRACT

The invention relates to a device comprising a matrix made of III-V nitride, said matrix comprising at least an active first portion through which an electrical current passes and at least a passive second portion through which no electrical current passes, said matrix comprising at least a first zone forming a first quantum confinement region made of a III-V nitride, said first zone being positioned in said active first portion, and at least a second zone forming a second quantum confinement region made of III-V nitride, characterized in that said second zone is positioned to said passive portion of said matrix.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a National Phase of Application No. PCT/FR2007/050898 filed Mar.9, 2007, which claims priority to French Application No. 06/50842, filedMar. 13, 2006; both of which are incorporated by reference herein.

BACKGROUND AND SUMMARY

The present invention relates to the field of light-emitting diodes, andmore particularly the field of monolithic light-emitting diodes.

Such diodes are, for example, known from the application U.S. Pat. No.6,445,009, wherein a diode is disclosed, which comprises a deviceincluding a substrate whereon a matrix is positioned, which comprises atleast a stack of quantum wells of the GaN or GaInN type emitting in thevisible spectrum at an ambient temperature in a layer of AlN or GaNrespectively. In the application U.S. Pat. No. 6,445,009, a system of(Al, Ga, In)N or III-V nitrides materials is used, and these aresemiconductors having a wide bandgap, and which have the characteristicof being able to emit in the whole visible spectrum. For example, awhite light is obtained by mixing in the active zone of the diode, i.e.in the zone through which the current of the diode passes, quantum wellsor InGaN/(AI)GaN quantum dots emitting in the blue and other onesemitting in the yellow.

Besides, it is also admitted that III-V nitrides are efficient materialsfor the production of monolithic light-emitting diode. However, it isknown that the output of emissions through electric pumping variesaccording to the emitted wavelength and more particularly, that theoutput in the blue is twice as good as the output in the yellow. This isthe reason why the diodes emitting in the blue are the most widely usedon the market today. The light output in the diode emitting in the blueand in the yellow is thus limited by the properties of the quantum dotsor the quantum wells emitting in the yellow.

A first aim of the present invention is not to be limited by lightoutputs through an electric injection at different wavelengths. Inaddition, it is known that in the electric injection diodes usingquantum dots or quantum wells, the distribution of the electrons and theholes in the quantum dots or in the quantum wells is modified as afunction of the voltage applied to the diode. The colour emitted maythus vary with the intensity of the electric current. Thischaracteristic may be useful if diodes which can change colours aredesired, but it is a drawback for the emission in the white when thepurpose is lighting.

A second aim of the present invention is thus to avoid the variation ofthe colour emission as a function of the intensity of the current of thediode. In the field of non monolithic diodes, white diodes are widelyused, which are composed of a first monolithic part emitting in the blue(blue diode), above which a phosphorescent material is positioned whichabsorbs a part of the blue photons emitted by the blue diode and reemitsyellow photons, the combination of such two lights giving a white light.

However, such two-part diodes of the standard blue diode—phosphor typehave, on the one hand, the drawback that the phosphor positioned abovethe blue diode results in a deterioration of the general performances ofthe device over time. This leads to a deterioration of the white lightover time and to a limitation of the life of the white LED with respectto a standard blue LED. On the other hand, it should be noted that themanufacturing of such a white LED requires a first conventional step ofepitaxial growth of the blue LED and an additional step of thedeposition of phosphor. Such additional step of the deposition thusinvolves higher production costs.

Another aim of the present invention is thus to provide a white diodethrough epitaxial growth only, i.e. a monolithic diode. Another aim ofthe present invention is thus to provide a white diode, the emissionproperties of which are stable over time.

The application US 2003/006430 which discloses a diode comprising Si- orSe-doped GaN layers making it possible to obtain an emission in theyellow. It is also known that in such layers, the emission is caused bydeep levels resulting from crystalline defects. The quantum efficiencyof this type of layer is thus limited. Besides, such deep levels emit ata wavelength which is fixed in the yellow. The diode of theabove-mentioned document does not make it possible to obtain a diodewhich can emit in the whole visible spectrum by combining the coloursbetween the light emitted in the active zone and the light emitted inthe passive zone.

The application 2002/0139984 is also known, which describes a diodecomprising a stack of semi-conductive layers in the passive zone of thediode. Such stack of layers is made so as to compose a mirror for thewavelength emitted by the active zone, in order to increase the photonsextraction efficiency. Besides, such layers must have an emissionwavelength very close to that of the active zone for example less than0.9 times half the width of the spectrum. Thus, for example for a blueLED at 450 nm and a width of 20 nm, the emission wavelength of suchlayers is comprised between 450 and 459 nm. The result is that thefunction of the semi-conductive layers in the above-mentioned documentis not to denature the colour of the active zone, so that for example ablue LED remains blue, while increasing the extraction efficiency byusing the mirror properties of the stack of quantum wells. Such a diodethus does not make it possible to make colour combinations between thelight emitted by the active zone and the light emitted by the passivezone.

Another aim of the invention is to provide a diode which can potentiallyemit in the whole visible spectrum and more particularly when it isexcited by a blue LED. At least one of these aims is reached accordingto the present invention by a device comprising a matrix made of III-Vnitride, said matrix comprising at least an active first portion throughwhich an electric current passes and at least a passive second portionthrough which no electric current passes, said matrix comprising atleast a first zone comprising a first stack of quantum wells or planesof quantum dots of III-V nitride, said first zone being positioned insaid active first portion and at least a second zone comprising a secondstack of quantum wells or planes of quantum dots of III-V nitride,characterised in that said second zone is positioned in said passiveportion of said matrix. Thanks to the stacks of quantum wells or quantumdots respectively positioned in the active portion and the passiveportion of the device, it is possible to control efficiently the lightsemitted by the first zone and the second zone in order to generate, atthe device output, a light which can spread on the whole visiblespectrum.

According to the invention, the first zone comprising a first stack ofquantum wells or planes of quantum dots of III-V nitride forms a firstquantum confinement and a second zone comprising a second stack ofquantum wells or planes of quantum dots of III-V nitride forms a secondquantum confinement. By positioning the second quantum confinement zonein the passive portion of the monolithic matrix, the present inventionsolves the problems relating to the difference in the emission as afunction of the currents which go through two different quantum zones inthe known diodes. Thus, in the device according to the presentinvention, the second quantum confinement zone positioned in the passivezone will in fact be optically pumped by the photons emitted by thefirst quantum confinement zone, the latter being electrically pumped bythe current of the diode passing in the active zone.

The optical pumping of the second zone in the passive portion of thematrix of the device thus makes it possible to avoid the drawbacksconnected to the electric pumping, and more particularly the dependenceof the emission on the current intensity. Besides, the monolithicconstitution of the matrix comprising the passive zone and the activezone in one III-V nitride material, makes it possible to obtain thedevice according to the invention with only one step of epitaxialgrowth. The distribution of the III-V nitride elements in the matrix iscarried out according to the invention, so that said first zone forms aquantum confinement and that the second zone forms a quantumconfinement, i.e. the part of the matrix between such zones forms aquantum gate between such zones. This is obtained, in a manner known perse, by selecting the III-V nitride materials as a function of thebandgaps of such materials.

In order to be able to adapt, in particular, the colour of the lightemitted by the first zone, said at least one first zone is able to emitphotons at at least a first wavelength through an electric injection bysaid current passing in said active zone, said at least first wavelengthbeing determined by the dimensions of said first stack of quantum wellsor planes of quantum dots of III-V nitride and the composition of saidfirst stack of quantum wells or planes of quantum dots of III-V nitride.Similarly, said at least second zone is able to emit photons at least asecond wavelength through an optical pumping by said photons emitted bysaid first zone, said at least one second wavelength being determined bythe dimensions of said second stack of quantum wells or planes ofquantum dots of III-V nitride and the composition of said second stackof quantum wells or planes of quantum dots of III-V nitride.

In order to diffuse a white light and more particularly for lightingapplications, said first zone and said second zone are selected so thatthe combination of the light signal corresponding to the photons at saidwavelength and the light signal corresponding to the photons of saidsecond wavelength diffuses a substantially white light. In order toreach a good output of light emissions, said at least one first zone isable to emit photons in the blue through an electric injection by saidcurrent passing in said active zone. In this case, in order to obtain awhite light at the output of the device, said at least one second zoneis able to emit photons in the yellow by an optical pumping by saidphotons emitted by said first zone. The emission in the yellow makes itpossible to use only one stack of quantum wells or planes of quantumdots.

According to another embodiment, which makes it possible to improve thecolour temperature and the colour rendering index while obtaining awhite light at the device output, said at least one second zone is ableto emit photons in the green and in the red by an optical pumping bysaid photons emitted by said first zone. According to an alternativesolution of the invention, said first zone is composed of a stack ofquantum wells of the InGaN/GaN type. In such embodiment of theinvention, said second zone is composed of a stack of planes of quantumdots of the GaN/AlN type. In order to allow the passage of the currenttowards the first active zone while making the quantum confinement ofthe first active zone, said matrix made of III-V nitride includes athird conductive portion which forms a quantum gate for said first zone.

The invention also relates to a light-emitting diode comprising a devicesuch as previously described and means for generating said current, saidmeans for generating said current being so arranged as to define saidactive first portion through which said electric current passes and saidpassive second portion through which no electric current passes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood upon reading the followingdetailed description and referring to the appended figures wherein:

FIG. 1 shows an exemplary light-emitting diode according to the presentinvention;

FIG. 2 shows a chromatic diagram for the selection of the components ina light-emitting diode according to FIG. 1;

FIG. 3 shows the emission wavelength in nanometres in an AlN/GaN/AlNquantum well or for an AlN/GaN/AlN quantum dot which can be used in adevice such as the one shown in FIG. 1;

FIG. 4 shows the emission wavelength for a Ga_(1-x)In_(x)N thick layerwhich can be used in the device of FIG. 1;

FIG. 5 shows the emission wavelength for Ga_(0.8)In_(0.2)N wells whichcan be used in the device of FIG. 1;

FIG. 6 shows another exemplary embodiment of a light-emitting diodeaccording to the invention; and

FIG. 7 shows still another exemplary embodiment of the light-emittingdiode according to the invention.

DETAILED DESCRIPTION

As illustrated in FIG. 1, a light-emitting diode 1 according to theinvention includes a substrate 7 on which a matrix 13 is formed whichcomprises various portions which will be described in further detailshereinunder. The matrix 13 first includes an active portion conductingelectric current and more particularly current lines 11. Such electriccurrent is generated by current generating means, for example, by metalcontacts in the form of a positive terminal 10A and a negative terminal10B and a semitransparent contact 10C. The active portion of the matrix13 is thus defined as the portion of the matrix through which thecurrent emitted by the current generating means 10A, 10B and 10C passes.Such active portion comprises a first zone 3 positioned in the activeportion so as to be gone through by the current lines 11. Such zone 3corresponds to a quantum confinement able to be electrically pumped bythe current emitted by the current generating means 10A, 10B and 10C. Itwill be called hereinunder an electric injection zone. Such electricinjection zone 3 is for example composed of a stack of InGaN/GaN quantumwells. The characteristics of such electric injection zone 3 areselected so that the electric injection made by the current enables theemission of photons, for example blue photons, as shown by the arrow 8.Such electric injection zone 3 is limited by a n-doped GaN conductivepart 4 and a p-doped GaN part 12. The function of such parts 4 and 12 isto conduct the current through the confined zone 3. The currentgeneration means 10A, 10B and 10C, the active portion and the quantumconfinement electric injection zone 3 generally form a blue diode 2, thestructure of which is known per se.

According to the invention, the matrix 13 also comprises a passiveportion which cannot be reached by the electric field lines 11.According to the invention, a quantum confinement zone 5 is positionedin this passive portion. Such zone 5 is, for example, composed of astack of GaN/AlN quantum dots. This zone 5 may be optically pumped bythe photons emitted by the electric injection zone 3. The zone 5 will becalled hereinunder an optical pumping zone.

Thanks to the optical pumping by the photons emitted by the electricinjection zone 3, the optical pumping zone 5 emits photons at awavelength which is different from that it receives. More precisely, ina manner known for the optical pumping, the length of the photonsemitted by the optical pumping zone 5 is greater than the wavelength ofthe excitation photons it receives from the electric injection zone 3.The passive portion also comprises an AlN zone 6 having the function ofa quantum gate for the GaN quantum dots of the optical pumping zone. Theheight of the GaN quantum dots of the optical pumping zone 5 is selectedso that the photons 9 emitted by this zone have a wavelengthsubstantially in the yellow. The number of planes of quantum dots isselected so that some blue photons 8 pass through the optical pumpingzone 5. The optical pumping zone z then has the function of ablue/yellow passive converter. The combination of the blue photons 8emitted by the electric injection zone 3 and the yellow photons 9emitted by the optical pumping zone 5 then enable the generation of asubstantially white light at the output of a transparent substrate 7,for example made of sapphire.

The emissions properties of the electric injection zone 3 and theoptical pumping zone 5 may be adapted to the desired colour by modifyingthe dimensions and compositions of the quantum wells and the quantumdots of at least one of these zones. More particularly, all thecombinations of colours such as given on the chromatic diagram in FIG. 2are possible. For lighting purposes, a combination of colours generatinga white light will be chosen, but it should be noted that the colouredlight may result from the combination of wavelengths emitted by thefirst electric injection zone 3 and the second optical pumping zone 5 aspreviously described. More particularly, in order to improve the colourtemperature and the colour rendering index of the colours obtained, thecombination of a first stack emitting in the red and the second stackemitting in the green, with these two stacks still being, according tothe invention, positioned in a passive zone of the matrix 13 of thediode 1, may be preferred to a stack of quantum dots emitting in theyellow.

It is of course understood that various combinations of the III-Vnitride materials may be used in the constitution of the matrix 13according to the invention, while respecting the obligations of quantumconfinement for the electric injection 3 and optical pumping 5 zones.The III-V nitride are GaN, InN, AlN and the alloys thereof and can benoted (Ga, In, Al)—N. More particularly, GaN quantum dots can be used inAlN or GaInN quantum dots can be used in AlN or GaInN quantum dots canbe used in AlGaInN. GaInN/GaN quantum wells can also be used. It is alsoknown that all such combinations comply with the conditions of thequantum confinement. Using only III-V nitrides for the matrix 13 makesit possible to make a diode 1 with only one step of epitaxial growth andto take advantage of the correct light emission properties of suchmaterials.

Now, we are giving a detailed description of an exemplary structure anddimension of the matrix 13 according to the invention in the table 1herein under which shows the succession of layers in the matrix 13according to the invention.

TABLE 1 Layer composition Typical thickness (nm) Reference GaN:Mg 200 12Al_(0.1)Ga_(0.9)N:Mg 10 12 GaN 7.5 3 Ga_(0.85)In_(0.15)N quantum wells2.5 3 GaN:Si 2,000 4 GaN nid 1,000 4 AIN 10 5 Plane of GaN quantum dots3 5 AIN 300 6 GaN buffer layer 30 7 Sapphire Substrate 350,000 7

In the table 1 hereabove, the references of the third column correspondto the references of FIG. 1. The first electric injection zone 3 iscomposed of five stacks of Ga_(0.85)In_(0.15)N quantum wells and onelayer of GaN. The second optical pumping zone is composed of twentystacks of planes of GaN quantum dots and one layer of AlN. The exemplarydimension and composition given in the table 1 make it possible to emitin the blue through an electric injection of zone 3, and the emission inthe yellow by the optical pumping of zone 5.

Besides, other combinations of compositions and dimensions may becarried out by the person skilled in the art, more particularly usingemission graphics of the FIGS. 3, 4 and 5. FIG. 3 shows the emissionwavelength in nanometre in an AlN/GaN/AlN quantum well or for anAlN/GaN/AlN quantum dot. The emission wavelength depends on the width ofthe quantum well or the dimensions of the quantum dots, moreparticularly the height of the quantum dots.

FIG. 4 shows the emission wavelength for a Ga_(1-x)In_(x)N thick layerwithout any quantum effect. Such wavelength more particularly depends onthe x component in In. Eventually, FIG. 5 shows the emission wavelengthfor Ga_(0.8)In_(0.2)N quantum wells in a GaN matrix.

Now, various alternative solutions of the invention will be described.As illustrated in FIG. 6, the quantum of confinement optical pumpingzone 5 may also be positioned above the quantum confinement electricinjection zone 3 in the direction of the epitaxial growth. In suchconfiguration, the metal contacts 10C have been eliminated so as todirect the field lines 11 while avoiding an upper passive portioncontaining the optical pumping zone 5. In operation, the electricinjection zone 3 is electrically pumped by the current according to thecurrent lines 11 so as to emit photons, for example in the blue, to thetop of the device. Such photons then optically pump the optical pumpingzone 5 which re-emits the photons, for example in the yellow. The lightobtained through the combination of such lights may then, as previouslymentioned, be a white light.

As illustrated now in FIG. 7, it is also possible to combine bothembodiments such as previously described in one diode 1 comprising afirst quantum confinement optical pumping zone 5A and a second quantumconfinement optical pumping zone 5B, both being positioned in a passiveportion of the matrix of diode 1. The first optical pumping zone 5A is,for example, positioned above the electric injection zone 3, and thesecond optical pumping zone 5B is positioned thereunder. As mentionedpreviously, the current generation means 10A and 10B are so arrangedthat no current goes through the passive portion of the matrix. Bothoptical pumping zones 5A and 5B may be chosen for emitting photons atthe same wavelength or at different wavelengths. According to anothernot shown embodiment, it is also possible to position several quantumconfinement electric injection zones 3 in the active portion of thematrix 13 gone through by an electric current.

In every embodiment, it should be noted that the person skilled in theart will have the possibility of adapting the parameters of thethickness of the optical or electrical pumping zones, so as to obtainthe light emission requested by combining the photons emitted by bothoptical and electrical pumping zones. He will also be able to obtain awhite light by using the teaching relating to the combination of colourssuch as illustrated in FIG. 2. The combination of an emission of thefirst zone 3 in the blue and planes of quantum dots emitting in the redand in the green more particularly makes it possible to obtain a correctcolour temperature as well as a correct colour rendering index.

1. A device comprising a matrix made of III-V nitride, said matrixcomprising at least an active first portion through which an electriccurrent passes and at least a passive second portion through which noelectric current passes, said matrix comprising at least a first zoneforming a first stack of quantum wells or planes of quantum dots ofIII-V nitride, said first zone being positioned in said active firstportion and at least a second zone comprising a second stack of quantumwells or planes of quantum dots of III-V nitride, said second zone ispositioned in said massive portion of said matrix.
 2. A device accordingto claim 1, wherein said at least first zone is able to emit photons atleast a first wavelength through an electric injection by said currentpassing in said active zone, said at least first wavelength beingdetermined by the dimensions of said first stack of quantum wells orplanes of quantum dots of III-V nitride and the composition of saidfirst stack of quantum wells or planes of quantum dots of III-V nitride.3. A device according to claim 2, wherein said at least second zone isable to emit photons at least a second wavelength through an opticalpumping by said photons emitted by said first zone, said at least secondwavelength being determined by the dimensions of said second stack ofquantum wells or planes of quantum dots of III-V nitride and thecomposition of said second stack of quantum wells or planes of quantumdots of III-V nitride.
 4. A device according to claim 3, wherein said atleast first zone and said at least second zone are selected so that thecombination of the light signal corresponding to the photons at said atleast one first wavelength and of the second light signal correspondingto the photons at said at least one second wavelength diffuses a whitelight.
 5. A device according to claim 1, wherein said at least one firstzone is able to emit photons in the blue through an electric injectionby said current passing in said active zone.
 6. A device according toclaim 5, wherein said at least one second zone is able to emit photonsin the yellow through an optical pumping by said photons emitted by saidfirst zone.
 7. A device according to claim 5, wherein said at least onesecond zone is able to emit photons in the green and in the red throughan optical pumping by said photons emitted by said first zone.
 8. Adevice according to claim 1, wherein said first zone comprises a stackof quantum wells of the InGaN/GaN type.
 9. A device according to claim1, wherein said second zone comprises a stack of planes of quantum dotsof the GaN/AlN type.
 10. A light-emitting diode comprising a device madeof III-V nitride, and means for generating electric said current, saidcurrent generating means being arranged as to define an active firstportion through which said electric current passes and an passive secondportion through which no electric current passes.